Wallace effect
Encyclopedia
The Wallace Effect is a hypothesis developed by British
naturalist
Alfred Russel Wallace
which posits that natural selection
can contribute to the reproductive isolation
of incipient species
by encouraging varieties to develop barriers to hybridization.
in On the origin of species. In it he proposed that natural selection could cause the reproductive isolation
of two varieties by encouraging the development of barriers against hybridization, and thus contribute to the development of new species. He suggested the following scenario: When two varieties of a species had diverged beyond a certain point, each adapted to different conditions, hybrid offspring would be less well adapted than either parent form. At that point natural selection will tend to eliminate the hybrids. Under such conditions natural selection would also favor the development of barriers to hybridization, since individuals that avoided hybrid matings would tend to have more fit offspring. This would contribute to the reproductive isolation of the two incipient species.
, especially sympatric speciation
. Its validity has been supported by mathematical models, and recently by empirical field data on the evolution of differing flowering times as a reproductive isolation mechanism, as well as sex-chromosome linked
species preference in flycatchers
.
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
naturalist
Naturalist
Naturalist may refer to:* Practitioner of natural history* Conservationist* Advocate of naturalism * Naturalist , autobiography-See also:* The American Naturalist, periodical* Naturalism...
Alfred Russel Wallace
Alfred Russel Wallace
Alfred Russel Wallace, OM, FRS was a British naturalist, explorer, geographer, anthropologist and biologist...
which posits that natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
can contribute to the reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
of incipient species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
by encouraging varieties to develop barriers to hybridization.
Historical development
In Wallace's 1889 book Darwinism, he explained and defended the theory of natural selection, first widely presented in 1859 by Charles DarwinCharles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
in On the origin of species. In it he proposed that natural selection could cause the reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
of two varieties by encouraging the development of barriers against hybridization, and thus contribute to the development of new species. He suggested the following scenario: When two varieties of a species had diverged beyond a certain point, each adapted to different conditions, hybrid offspring would be less well adapted than either parent form. At that point natural selection will tend to eliminate the hybrids. Under such conditions natural selection would also favor the development of barriers to hybridization, since individuals that avoided hybrid matings would tend to have more fit offspring. This would contribute to the reproductive isolation of the two incipient species.
Current status
Sometimes also called "reinforcement" or "heterozygote disadvantage", the Wallace effect continues to be a topic of research in evolutionary biology, as it is potentially an important factor in speciationSpeciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
, especially sympatric speciation
Sympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
. Its validity has been supported by mathematical models, and recently by empirical field data on the evolution of differing flowering times as a reproductive isolation mechanism, as well as sex-chromosome linked
Sex linkage
Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance...
species preference in flycatchers
Collared Flycatcher
The Collared Flycatcher, Ficedula albicollis, is a small passerine bird in the Old World flycatcher family, one of the four species of Western Palearctic black-and-white flycatchers. It breeds in southeast Europe and southwest Asia and is migratory, wintering in sub Sahara Africa...
.